The molecular basis of prostate cancer is becoming increasingly elucidated as the genetic abnormalities underlying the disease are identified. Most prominent among these is the androgen receptor (AR), which for decades has been known to be essential for the survival of prostate cancer cells. However, because transcription factors (including AR) are generally considered 'undruggable', pharmacological approaches to modulating AR activity have been limited, focusing on blocking ligand binding to AR, and hormonally diminishing the body's production of androgens. While tumors initially respond to such androgen deprivation therapy, recurrence inevitably occurs. New approaches to modulating AR activity are therefore needed. Similarly, the recently identified mutations in the ETS family of transcription factors observed in as many as 70% of prostate cancers call for a novel approach to pharmacologically inhibiting the activity of these mutant oncoproteins. During the prior funding cycle, we developed a novel, gene expression-based chemical screening method (GE-HTS), and applied it to the discovery of AR-modulating small molecules. Most prominent among the hits that emerged from that screen were a group of structurally unique natural products that we demonstrated were functioning as novel HSP90 inhibitors. Based on these preliminary findings, we propose the following Specific Aims: Aim 1. Test the hypothesis that HSP90 inhibition will abrogate AR function and result in clinical responses in patients with advanced prostate cancer. Aim 2. Develop a signature of TMPRSS2/ERG activity. Aim 3. Screen small molecule libraries to identify compounds capable of modulating the TMPRSS2/ERG signature. Aim 4. Validate the hits emerging from the TMPRSS2/ERG GE-HTS screen.